A. Field of the Invention
The direct excitation of large lasers by sunlight offers the prospect of a drastic reduction in the cost of coherent optical radiation for high average power applications. Up to now solar pumped lasers have not been viewed from this perspective. Solar optical excitation of the Nd:YAG laser has been developed for laser communication between space satellites. However, these small, low power devices (1/2 watt) have severe cooling requirements due to their low efficiency and are not suitable for scaling to high power. By contrast, this invention consists of a direct solar pumped laser capable of high average power output. In direct solar excitation, the laser itself is the energy conversion device. The spectrum of incident sunlight is an essentially thermal black body distribution (approximately 5800 K) modified by atmospheric scattering and absorption. The energy output is coherent light, which has a lower entropy, thus producing a higher form of energy. Large direct solar pumped lasers could be used for present laser applications in industrial machining and heating operations. Their use in industrial photochemistry and as optical excitation sources for other lasers would greatly enlarge the scale of these applications. Any significant reduction in the cost of laser light will not only expand the range of existing applications but will also open up entirely new ones. For example, new laser developments would evolve, since large volumes of high pressure gas (.gtoreq.10 atm) can be efficiently pumped by laser light. Other examples include photovoltaic cells designed for optimum performance at a single optical frequency, which have significantly increased efficiency (.about.40%). Multijunction solar cells are edge illuminated and have the practical advantage of high output voltage. With incident light that is coherent, it becomes possible to focus the illumination so that it propagates only through the junction region of the cell. Not only would conversion efficiency be further enhanced, but the allowable light concentration would be dramatically increased. Heating due to recombination would be nearly eliminated. Moreover, when the semiconductor bandgap and the laser frequency are matched, heating from excess photon energy is greatly reduced. Multijunction solar cells might then be operated very efficiently at high power levels. Such developments would radically alter the solar photovoltaic concept from the current large arrays of essentially planar cells exposed to outdoor conditions to compact, high power "volume" devices that can be kept in a controlled environment because direct solar irradiance has been eliminated. Even more appealing is the application of solar powered lasers to the photoelectrolysis of water, where the generation of hydrogen automatically solves the energy storage problem. A coherent light beam can be focused to the high intensities at which nonlinear effects such as second harmonic generation are large enough to be used efficiently. The dissociation of water into hydrogen and oxygen by photoelectrolysis occurs at a much higher efficiency with light in the near uv compared to the visable. The frequency-doubled output from a solar pumped laser would provide an optimum irradiation source for the process. Probably the most exciting prospect for solar pumped lasers is the economical generation of hydrogen by photoelectrolysis.
To obtain a match between solar energy and the excitation of a laser system, two properties of sunlight are critical, namely, its broad continuous frequency distribution and its constant intensity over time intervals characteristic of laser action. With such a diffuse spectrum, only a small fraction of the total irradiant power resides in any small frequency interval. The active molecule of this laser invention must have an electronic absorption transition that lies in the intense (visible) part of the solar spectrum with a large total oscillator strength distributed over a wide continuum band. The steady intensity of the solar light source in time means that the medium must be pumped continually for highest efficiency. Efficient energy extraction dictates CW (continuous wave) or fast repetitively pulsed operation of the laser. Compounds having high absorption in the visible spectrum are generally called dyes. These are long organic molecules possessing several conjugated double bonds. The longest wavelength, spin allowed absorption band, S.sub.1 .rarw.S.sub.0, of a dye molecule will generally meet the inventor's absorption requirements, especially in solution at normal temperature where the close-spaced vibrational structure of a vibronic band system is broadened into a continuum band with one or at most a few relatively broad peaks. Using this criteria, two classes of dye molecules chosen were aromatic hydrocarbons and organic dyestuffs. Organic dyestuff molecules contain a chromophoric system, characterized by a chain of conjugated double bonds within which the desired electronic transition takes place.
B. Prior Art
1. "Organic Laser Systems" in Lasers, vol. 1, pp. 203-211, 1966.
A. Lempicki et al. discuss the use of the phosphorescence transition of organic molecules for laser operation with flashlamp excitation. Coherent stimulated emission from the triplet state of aromatic organic phosphors had been reported, however, efforts to reproduce the reported results were not successful. These molecules were not suited for solar pumping because they absorb in the ultraviolet which comprises a small fraction of the solar spectrum.
2. U.S. Pat. No. 3,451,010, T. H. Maiman. FIG. 3 shows a means for optically pumping the laser material with sunlight energy using a lens and mirror to focus the light on the material.
3. Optics Letters, B. A. Swartz et al., vol. 1, pp. 73-75, August 1977. Multiple dye planar solar concentrators have been demonstrated using a plastic matrix.
4. Opt. Spektrosk, S. G. Rautian et al., vol. 10, pp 65-66, January 1961. Heavy-atom substitution to increase the quantum yield for phosphorescence in phosphorescence laser systems is suggested, but not in connection with the explicit use of and substitution in dye molecules.
In this invention suitable dye molecules are selected to have little or no absorption of laser emission. This invention is a direct solar pumped laser capable of high power output. A planar solar concentrator can be used as the sunlight collector to supply the pump light to the lasing medium.